Electrode feed arrangements

ABSTRACT

An arrangement for controlling the electro-slag process of producing ingots in which the actual growth of the ingot is compared with a required growth and a difference signal produced and utilized to vary the applied voltage so that the electrode is consumed at a rate which will give the required growth of the ingot.

United States Patent Inventor Appl. No.

Filed Patented Assignee Priorities Alec George Allen Sutton Coldiield, England May 14, 1969 Nov. 23, 1971 Associated Electrical industries Limited London, England May 14, 1968 Great Britain Sept. 17, 1968, Great Britain, No. 44,149/68; Jan. 14, 1969, Great Britain, No. 2,095/69 ELECTRODE FEED ARRANGEMENTS 10 Claims, 5 Drawing Figs.

U.S.Cl 13/13 Int. Cl ..F27d 1 l/ 10, l-l05b 3/60 Primary Examiner-Bernard A. Gilheany Assistant Examiner-R. N. Envail, Jr. Attorney-Larson, Taylor and Hinds ABSTRACT: An arrangement for controlling the electro-slag process of producing ingots in which the actual growth of the ingot is compared with a required growth and a difference signal produced and utilized to vary the applied voltage so that the electrode is consumed at a rate which will give the required growth of the ingot.

PATENTEDNUV 23 197i 3,622,678

SHEET 1 OF 5 ELECTRODE FEED ARRANGEMENTS This invention relates to control arrangements suitable for use with electro-slag-refining apparatus.

In an electro-slag-refining apparatus the depth and shape of the pool of molten metal at the top of the solidifying ingot has a pronounced efiect on the structure and quality of the ingot. Ideally, the pool of molten metal should have a shallow dish shape so that an ingot of homogeneous structure is produced and alloy segregation is avoided. The principal factors controlling the depth and shape of the pool of molten metal are the rate at which the molten metal is laid down, i.e. the rate of growth of the ingot, the rate of cooling of the molten metal brought about by mold wall and the mold base and the separation between the lower end of the electrode and the top of the ingot. The rate of growth of the ingot depends on the rate of consumption of the electrode which is in turn affected by the power dissipated at the electrode. When the effective melting power at the electrode increases, due to an increase in the supply voltage or to a decrease in the thermal losses of the system, molten metal is deposited on the top of the ingot relatively quickly and the area over which heat transfer takes place needs to be increased in order to maintain stable thermal conditions. Since the cooling capacity of the mold wall and mold base remain unchanged, the area over which heat transfer takes place can be increased only by an increase in the area over which solidification takes place, so that the pool of molten metal tends to deepen and take on an inverted bell shape. With apool of molten metal of this inverted bell shape, alloy segregations appear and impurities and alloy segregations tend to become trapped at the center of the solidifying part of the ingot.

An object of the invention is to provide an arrangement for controlling the rate of growth of the ingot and/or the separation between the lower end of the electrode and the top of the ingot so that the depth and shape of the pool of molten metal can be kept within desired limits.

According to one aspect of the present invention a control arrangement suitable for use with electro-slag-refining apparatus comprises, means for causing relative movement between a consumable electrode depending into a slag pool and a mould which contains the slag pool and in which an ingot is to be formed, means for applying a voltage between the electrode and the forming ingot, means for comparing the actual growth of the ingot with a required growth and providing a signal indicative of the difference therebetween, and means for varying the applied voltage in response to the difference signal so that the electrode is consumed at a rate which will give the required growth of the ingot.

The control arrangement may also include means for varying the relative movement between the electrode and the mold so that a predetermined separation is maintained between the lower end of the electrode and the top of the forming ingot.

The means for comparing the actual growth of the ingot with a required growth and providing a difference signal may include one or more suitable sensing devices disposed on the sides of the mold and arranged to monitor the position of the top of the ingot, or may include a load cell arranged to monitor the weight of the growing ingot, or may include a probe or probes arranged to monitor the position of the top of the ingot. The sensing devices referred to above may comprise thermocouples, capacitive sensing devices, X-ray apparatus, inductive sensing devices, devices responsive to radioactive material present in the metal from which the ingot is being formed and devices utilizing sonic or ultrasonic oscillations.

According to another aspect of the invention a control arrangement suitable for use with electro-slag-refining apparatus comprises, means for causing relative movement between a consumable electrode depending into a slag pool and a mold which contains the slag pool and in which an ingot is to be formed, means for applying a voltage between the electrode and the forming ingot, means for providing a reference signal indicative of the separation between the lower end of the electrode and the top of the ingot, and means for varying the applied voltage in response to the reference signal so that the electrode is consumed at a rate which will maintain a predetermined separation between the lower end of the electrode and the top of the growing ingot.

The reference signal may be provided by monitoring the voltage between the lower end of the electrode and the top of the ingot and providing a first signal indicative thereof, monitoring the current flowing between the electrode and the ingot and providing a second signal indicative thereof, and combining the first and second signals to provide a signal indicative of the electrical impedance between the electrode and the ingot and thus of the separation between lower end of the electrode and the top of the ingot. The reference signal may also be provided by one or more suitable sensing devices disposed on the sides of the mold and arranged to monitor the position of the lower end of the electrode and the top of the ingot. The sensing devices may comprise thermocouples, capacitive sensing devices, X-ray apparatus, inductive sensing devices, devices responsive to radioactive material present in the metal from which the ingot is being formed and devices utilizing sonic or ultrasonic oscillations.

The relative movement between the electrode and the mold may comprise feeding the electrode into the slag pool at a predetermined rate.

Moreover, it is not essential that the relative movement between the electrode and the mold should consist of movement of the electrode towards the mold, and it is envisaged that the mold could be moved towards the electrode in the manner described in our copending Pat. application No. 47656/68.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. I is a side elevation, partly in section, of an electroslag-refining apparatus incorporating a control arrangement in accordance with the invention, and

FIGS. 2 to 5 are side elevations similar to FIG. 1 but showing modified forms of the control arrangement in accordance with the invention.

Referring in the first instance to FIG. 1, the electro-slagrefining apparatus comprises an annular water cooled mold 1 into which depends a consumable electrode 3 from which an ingot 5 is to be formed. The lower end of the electrode 3 is im' mersed in a slag pool 7 which lies above the ingot 5 being formed in the mold l, and the electrode 3 is arranged to be lowered into the slag pool 7 by a winch mechanism 9 driven by an electric motor 11 the speed of which is arranged to be variable. A supply of alternating current 13 connect between the electrode 3 and the bottom plate 15 of the mold l is arranged so that the voltage applied between the electrode 3 and the ingot can be varied either continuously or in small increments by a voltage control device 17, for example a tap changer on a transformer. 'A voltage-measuring device 19 is arranged to measure the voltage between the lower end of the electrode 3 and the top of the growing ingot 5, that is the voltage across the portion of the slag pool 7 between them, and to provide an output signal indicative of the value of the voltage. A currentmeasuring device 21 is arranged to measure the current flowing between the electrode 3 and the ingot 5 and to provide an output signal indicative of the value of the current. The output signals from the voltage-measuring device 19 and the currentmeasuring device 21 are fed to an impedance-monitoring device 23 which combines the two signals and provides an output signal indicative of the electrical impedance between the electrode 3 and the ingot 5. The output signal from the monitoring device 23 is fed to a control device 25 arranged to vary the speed of the motor 11 and hence vary the rate at which the winch mechanism 9 lowers the electrode 3 into the slag pool, so that the impedance between the electrode and the ingot and hence the separation between the electrode and the ingot is maintained at a predetermined value. A load cell 27 mounted on the arm 29 supporting the electrode 3 is arranged to monitor the weight of the electrode and to provide an output signal which is indicative of the decreasing weight of the electrode as it is consumed and hence indicative of the growth of the ingot 5. The output signal from the load cell 27 is fed to a comparator 31 arranged to compare the actual growth of the ingot 5 with a required growth and to provide a signal indicative of the difference between them. This difference signal is applied to the voltage control device 17 to vary the voltage between the electrode and the ingot so that the electrode is consumed at a rate which will give the required growth of the ingot.

In use of the electro-slag apparatus the supply of alternating current applies a voltage between the bottom plate and the electrode 3 to cause a current to flow between them and melt the lower end of the electrode which dips into the slag pool 7. As the electrode 3 is consumed and the molten metal deposited on the bottom plate 15 to form the ingot 5, the electrode 3 is lowered by the winch mechanism 9 at a rate which maintains a predetermined separation between the lower end of the electrode 3 and the top of the forming ingot 5. The impedance-monitoring device 23 combines the signals from the voltage-measuring device 19 and the current-measuring device 21 and provides an output signal which is applied to the control device 25 to vary the speed of the motor to maintain the predetermined separation between the lower end of the electrode 3 and the top of the growing ingot 5. The load cell monitors the diminishing weight of the electrode and provides an output signal indicative of the diminishing weight of the electrode and hence indicative of the rate of growth of the ingot 5 This output signal is fed to the comparator 31 which compares the output signal with a required growth rate which will cause the pool of molten metal 33 to have a shallow dish shape as shown, and provides an output signal indicative of the difference between the two rates of growth. This difference signal is applied to the voltage control device 17 to adjust the voltage between the electrode and the ingot 5 to a value such that the electrode is consumed at a rate which will give the required growth of the ingot to maintain the pool of molten metal 33 ofsubstantially shallow dish shape.

Although in the arrangement described above a load cell 27 arranged to monitor the weight of the ingot 5 is utilized to ascertain the actual growth of the ingot. in other embodiments of the invention a load cell 41 may be arranged to monitor the weight of the mold 1 and thus ascertain the increasing weight of the growing ingot 5 and hence the rate of growth of the ingot.

As shown in FIG. 4 the growth of the ingot 5 can be monitored by one or more sensing devices 43 disposed on the sides of the mold l and arranged to provide output signals indicative of the position of the top of the growing ingot 5. The output signals from the sensing devices 43 are applied to an amplifier 45 and fed to a comparator 31 which compares each output signal with a required growth rate such as will cause the pool of molten metal 33 to have a shallow dish shape as shown. The comparator 31 provides an output signal indicative of the difference between the two rates of growth and this difference signal is applied to the voltage control device 17 to adjust the voltage between the electrode 3 and the ingot 5 to a value such that the electrode is consumed at a rate which will give the required growth rate of the ingot 5 to maintain the pool of molten metal 33 of substantially shallow dish shape. Such sensing devices 43 may comprise thermocouples, capacitive sensing devices, X-ray apparatus, inductive sensing devices, devices responsive to radioactive material present in the metal from which the ingot is being formed, and devices utilizing sonic or ultrasonic oscillations. As shown in FIG. 5 the growth of the ingot may also be monitored by two probes 47 arranged to provide an output signal indicative of the position of the top of the growing ingot. The two probes 47 are arranged to complete an electrical circuit to a monitor 49 by way of the slag pool 7 so that as the slag rises up the probes 47 during the growth of the ingot 5 the impedance of the monitor circuit varies in accordance with the growth. The monitor 49 provides an output signal indicative of the rate of growth of the ingot 5 and this output signal is fed to the comparator 31 and compared with a required growth rate such as will cause the pool of molten metal 33 to have a shallow dish shape as shown. The comparator 31 provides an output signal indicative of the difference between the two rates of growth and this difference signal is applied to the voltage control device 17 to adjust the voltage between the electrode 3 and the ingot 5 to a value such that the electrode is consumed at a rate which will maintain the pool of molten metal 33 of shallow dish shape.

The electro-slag-refining apparatus shown in HO. 1, can be modified for energization from a three-phase supply, in which case the connection to the bottom plate 15 of the mold is not required and each of the three phases is connected to a separate consumable electrode 3. The electrodes 3 may be lowered into the slag pool 7 separately, each electrode having its own electrode feed mechanism and its own arrangement for maintaining a predetermined separation between the lower end of that electrode and the top of the ingot 5. Alternatively. the three electrodes may be lowered into the slag pool by a common electrode feed mechanism, a common arrangement being provided for maintaining the mean value of the separation between the lower ends of the electrodes and the top of the ingot within predetermined limits.

Referring now to F l0. 2, the electro-slag-refining apparatus comprises an annular water-cooled mold 1 into which depends a consumable electrode 3 from which an ingot 5 is to be formed. The lower end of the electrode 3 is immersed in a slag pool 7 which lies above the ingot 5 being formed in the mold l, and the electrode 3 is arranged to be lowered into the slag pool 7 by a winch mechanism 9 driven by an electric motor 11 the speed of which can be set at a predetermined constant value. A supply of alternating current 13 connected between the electrode 3 and the bottom plate 15 of the mold l is arranged so that the voltage applied between the electrode 3 and the ingot 5 can be varied either continuously or in small increments by a voltage control device 17, for example, a tap changer on a transformer. A voltage-measuring device 19 is arranged to measure the voltage between the electrode 3 and the ingot 5, that is the voltage across the portion of the slag pool 7 between them, to provide an output signal indicative of the value of the voltage. A current-measuring device 21 is arranged to measure the current flowing between the electrode 3 and the ingot 5 and to provide an output signal indicative of the value of the current. The output signals from the voltagemeasuring device 19 and the current-measuring device 21 are fed to an impedance-monitoring device 23 which combines the two signals and provides a reference signal indicative of the electrical impedance and thus of the separation between the lower end of the electrode 3 and the top of the ingot 5. The reference signal from the monitoring device 23 is applied to the voltage control device 17 in which it is utilized to adjust the voltage of the alternating current supply 13 so that the electrical power dissipated at the electrode 3 is such as will consume the electrode 3 at the same rate as it is being lowered by the winch mechanism 9, while maintaining a substantially constant predetermined separation between the lower end of the electrode 3 and the top of the ingot 5 being formed. The rate at which the electrode 3 is lowered into the slag pool 7 and the predetermined separation between the lower end of the electrode 3 and the top of the forming ingot 5 are suitably chosen to provide a molten metal pool 33 substantially conforming to the ideal shallow dish shape.

The reference signal indicative of the separation between the lower end of the electrode 3 and the top of the ingot need not be obtained by monitoring the current and voltage and may be provided by one or more sensing devices 43 disposed on the sides of the mold l and arranged to monitor the positions of the top of the ingot 5 and the lower end of the electrode 3. These sensing devices 43 may comprise thermocouples, capacitive sensing devices, X-ray apparatus, inductive sensing devices, devices responsive to radioactive material present in the metal from which the ingot is being formed and devices utilizing sonic or ultrasonic oscillations.

An advantage of this embodiment of the invention is that since the electrode 3 only needs to be lowered into the slag pool 7 at a predetermined rate which does not have to be varied continuously in response to variations in other parameters of the arrangement, the mechanism for controlling the heavy cumbersome electrode 3 and its supporting arm 29 can be simpler and less expensive than would otherwise be necessary. Moreover, hot topping of the ingot 5 can be carried out by progressively diminishing the rate of lowering of the electrode 3 into the mold 1, since the power-dissipated at the electrode 3 and hence the rate at which the electrode is consumed will be correspondingly diminished automatically.

When the electro-slag-refining apparatus shown in FIG. 2 is energized from a three-phase supply, the connection to the bottom plate of the mold l is not required and each of the three phases of the supply is connected to a separate consumable electrode 3. The three electrodes are fed into the slag pool 7 at a predetermined rate and the current flowing to the electrodes and the voltages between the electrodes are monitored and utilized to adjust the voltages of the three phases of the supply so that the electrical power dissipated at each electrode is such as will consume the electrode at the same rate as it is being fed into the slag pool while maintaining substantially contact depth of immersion of that electrode in the slag pool. The electrode may be lowered into the slag pool separately, each electrode having its own electrode feed mechanism and the voltage between each electrode and the ingot being controlled by a separate reference signal provided by control devices individual to that electrode. Alternatively, the three electrodes may be lowered into the slag pool by a common electrode feed mechanism and the voltages between the electrodes and the ingot controlled by a common reference signal indicative of the mean value of the individual separations between the three electrodes and the ingot.

Referring now to FIG. 3, the electro-slag-refining apparatus comprises an annular water-cooled mold 1 into which depends a consumable electrode 3 from which an ingot 5 is to be formed. The lower end of the electrode is immersed in a slag pool 7 which lies above the ingot 5 being formed, and the electrode is arranged to be lowered into the slag pool 7 by a winch mechanism 9 driven by an electric motor 11, the speed of which is arranged to be variable. A supply of alternating current 13 connected between the electrode 3 and the bottom plate 15 of the mold is arranged so that the voltage applied between the electrode 3 and the ingot 5 can be varied either continuously or in small increments by a voltage control device 17, for example, a tap changer on a transformer. A voltage-measuring device 19 is arranged to measure the voltage between the lower end of the electrode 3 and the top of the ingot 5, that is the voltage across the portion of the slag pool between them, and to provide an output signal indicative of the value of the voltage. A current-measuring device 21 is arranged to measure the current flowing between the electrode 3 and the ingot 5 and to provide an output signal indicative of the value of the current. The output signals from the voltage-measuring device 19 and the current-measuring device 21 are fed to an impedance-monitoring device 23 which combines the two signals and provides an output signal indicative of the electrical impedance between the electrode 3 and the ingot 5. The output signal from the monitoring device 23 is fed to a control device 25 arranged to vary the speed of the motor 11 and hence vary the rate at which the winch mechanism 9 lowers the electrode 3 into the slag pool 7, so that the electrical impedance between the electrode 3 and the ingot 5 and hence the separation between the lower end of the electrode and the top of the ingot is maintained at a predetermined value. An electrode feed-measuring device 35 is arranged to ascertain the rate at which the electrode 3 is fed into the slag pool and to provide an output indicative of the actual rate of feed. A further device 37 is arranged to provide an output indicative of a required rate of feed. The outputs from the two devices 35 and 37 are fed to a device 39 which compares the two rates of feed and provides an error signal indicative of the difference between the two rates of feed. The error signal is fed to the voltage control device 17 and utilized to control the value of the applied voltage so that the electrical power dissipated at the electrode is such as to consume the electrode at a rate which maintains the required feed rate of the electrode. The predetermined values of electrical resistance (and thus the electrode/ingot separation), and the required electrode feed rate can be suitably chosen to provide a molten metal pool 33 substantially conforming to the ideal shallow dish shape. The arrangement comprising the electrode feed measuring device 35 and the two devices 37 and 39 can be a mechanical arrangement, for example, a differential gear arrangement comprising two shafts driven at speeds which correspond to the actual feed rate and the required feed rate respectively, the two shafts being arranged to drive a third shaft at a speed corresponding to the difl'erence between the two rates of feed. The third shaft would then be arranged to actuate the voltage control device 17 to vary the voltage applied between the electrode 3 and the ingot 5. Alternatively, the arrangement for ascertaining and comparing the two rates of feed may be an electrical arrangement, for example, the device 35 may be a tachogenerator arranged to generate electrical signals indicative of the actual feed rate, the device 37 may be arranged to generate electrical signals indicative of the feed rate required to maintain the molten metal pool 33 of shallow dish shape and the device 39 may be arranged to compare these two electrical signals and produce a third electrical signal indicative of the difference between the two rates of feed.

When the electro-slag-refining apparatus shown in F IG. 3 is energized from a three-phase supply, the connection to the bottom plate 15 of the mold is not required and each of the three phases is connected to a separate consumable electrode. The currents flowing between each electrode 3 and the ingot 5 and the voltages between each electrode and the ingot are monitored to provide signals which are utilized to vary the electrode feed rate. A difference signal indicative of the difference between the actual and required electrode feed rates is then utilized to adjust the voltages of the three phases of the supply so that the electrical power dissipated at each electrode is such as will consume that electrode at a rate which maintains a predetermined separation between the lower end of the electrode and the top of the ingot. The electrodes may be lowered into the slag pool separately, each electrode having its own electrode feed mechanism and control devices. Alternatively, the three electrodes may be lowered into the slag pool together by a common mechanism controlled in accordance with the mean values of the voltages between the electrodes and the ingot and the mean values of the currents flowing between the electrodes and the ingots, and in accordance with the mean values of the actual and required feed rates and the mean value of the differences between them.

It is envisaged that hot topping of an ingot will be able to be carried out automatically by progressively diminishing the required electrode feed rate, since the rate of consumption of the electrodes will be automatically adjusted to maintain a predetermined separation between the electrodes and the ingot.

It is not essential that the relative movement between the electrode and the mold should consist of lowering the electrode into the mold, and it is envisaged that the mold could be moved upwards towards the electrode in the manner described in our copending Pat. application No. 47656/68.

Moreover it is not essential that the required growth of the ingot should be substantially unchanged throughout the formation of an ingot, nor that the predetermined separation between the lower end of the electrode and the top of the ingot should be unchanged throughout the process, and provision may be made to vary these parameters either automatically in accordance with a program or manually to deal with any contingency that might arise. For example, these parameters may be varied to cater for the rapidly diminishing cooling effect of the mold base during the early stages of the growth of an ingot, or to effect hot topping" during the final stages of the formation of an ingot.

lclaim:

l. Electro-slag-reflning apparatus comprising:

a. means for causing relative movement between a consumable electrode depending into a slag pool and a mold which contains the slag pool and in which an ingot is to be formed;

b. means for applying a voltage between the electrode and the forming ingot;

c. means for varying the relative movement between the electrode and the mold to maintain the separation between the electrode and the ingot at a predetermined value;

v d. means for comparing the actual growth of the ingot with a required growth which will give a shallow dish-shaped pool of molten metal at the top of the ingot and providing a signal indicative of the difference therebetween; and

e. means for varying the applied voltage in response to the difference signal so that the electrode is consumed at a rate which will give the required growth of the ingot.

2. Electro-slag apparatus as claimed in claim 1, wherein the means for comparing the actual growth of the ingot with the required growth and providing a difference signal include a load cell for monitoring the weight of the diminishing electrode.

3. Electro-slag apparatus as claimed in claim 1 wherein the means for comparing the actual growth of the ingot with the required growth and providing a difference signal include a load cell for monitoring the weight of the growing ingot.

4. Electro-slag apparatus as claimed in claim 1, wherein the means for comparing the actual growth of the ingot with the required growth and providing a difference signal include a probe arrangement for monitoring the position of the top of the growing ingot.

5. Electro-slag apparatus as claimed in claim 1, wherein the means for comparing the actual growth of the ingot with a required growth and providing a difference signal include a plurality of sensing devices disposed on a side of the mold for monitoring the position of the top of the growing ingot.

6. Electro-slag apparatus as claimed in claim 1, wherein the means for varying the relative movement between the electrode and the mold comprise means for providing a first signal indicative of the voltage between the lower end of the electrode and the upper end of the ingot, means for providing a second signal indicative of the current flowing between the electrode and the ingot, and means for combining the first and second signals and producing an output signal indicative ofthe electrical impedance between the lower end of the electrode and the top of the growing ingot, which output signal is utilized to vary the relative movement between the electrode and the mold.

7. Electro-slag refining apparatus comprising:

a. means for causing relative movement between a consumable electrode depending into a slag pool and a mold which contains the slag pool and in which an ingot is to be formed;

b. means for applying a voltage between the electrode and the forming ingot;

c. means for varying the relative movement between the electrode and the mold to maintain the separation between the electrode and the ingot at a predetermined value;

d. means for comparing the actual rate at which the electrode is fed into the slag pool with a required electrode feed rate which will give a shallow dish shaped molten metal pool at the top of the ingot and providing an error signal indicative of the difference between the two rates of feed; and

e. means for varying the applied voltage in response to the error signal so that the electrode is consumed at a rate which maintains both the required feed rate and the predetermined separation between the lower end of the electrode and the top of the ingot. 8. Electro-slag apparatus as claimed in claim 7, wherein the means for comparing the actual electrode feed rate with a required electrode feed rate and providing an error signal comprise a differential gear arrangement having two rotatable members driven at speeds corresponding to the actual feed rate and the required feed rate respectively and arranged to drive a third rotatable member at a speed corresponding to the difference between the two rates of feed.

9. Electro-slag apparatus as claimed in claim 7, wherein the means for comparing the actual electrode feed rate with a required electrode feed rate and providing an error signal comprise two devices arranged to generate electrical signals indicative of the actual feed rate and the required feed rate respectively, and a further device arranged to compare the two electrical signals and produce a third electrical signal indicative of the difference between the two rates of feed.

10. Electro-slag apparatus as claimed in claim 7, wherein the means for varying the relative movement between the electrode and the mold comprise means for providing a first signal indicative of the voltage between the lower end of the electrode and the upper end of the ingot, means for providing a second signal indicative of the current flowing between the electrode and the ingot, and means for combining the first and second signals and producing an output signal indicative of the electrical impedance between the lower end of the electrode and the top of the growing ingot, which output signal is utilized to vary the relative movement between the electrode and the mold. 

1. Electro-slag refining apparatus comprising: a. means for causing relative movement between a consumable electrode depending into a slag pool and a mold which contains the slag pool and in which an ingot is to be formed; b. means for applying a voltage between the electrode and the forming ingot; c. means for varying the relative movement between the electrode and the mold to maintain the separation between the electrode and the ingot at a predetermined value; d. means for comparing the actual growth of the ingot with a required growth which will give a shallow dish-shaped pool of molten metal at the top of the ingot and providing a signal indicative of the difference therebetween; and e. means for varying the applied voltage in response to the difference signal so that the electrode is consumed at a rate which will give the required growth of the ingot.
 2. Electro-slag apparatus as claimed in claim 1, wherein the means for comparing the actual growth of the ingot with the required growth and providing a difference signal include a load cell for monitoring the weight of the diminishing electrode.
 3. Electro-slag apparatus aS claimed in claim 1 wherein the means for comparing the actual growth of the ingot with the required growth and providing a difference signal include a load cell for monitoring the weight of the growing ingot.
 4. Electro-slag apparatus as claimed in claim 1, wherein the means for comparing the actual growth of the ingot with the required growth and providing a difference signal include a probe arrangement for monitoring the position of the top of the growing ingot.
 5. Electro-slag apparatus as claimed in claim 1, wherein the means for comparing the actual growth of the ingot with a required growth and providing a difference signal include a plurality of sensing devices disposed on a side of the mold for monitoring the position of the top of the growing ingot.
 6. Electro-slag apparatus as claimed in claim 1, wherein the means for varying the relative movement between the electrode and the mould comprise means for providing a first signal indicative of the voltage between the lower end of the electrode and the upper end of the ingot, means for providing a second signal indicative of the current flowing between the electrode and the ingot, and means for combining the first and second signals and producing an output signal indicative of the electrical impedance between the lower end of the electrode and the top of the growing ingot, which output signal is utilized to vary the relative movement between the electrode and the mold.
 7. Electro-slag refining apparatus comprising: a. means for causing relative movement between a consumable electrode depending into a slag pool and a mold which contains the slag pool and in which an ingot is to be formed; b. means for applying a voltage between the electrode and the forming ingot; c. means for varying the relative movement between the electrode and the mould to maintain the separation between the electrode and the ingot at a predetermined value; d. means for comparing the actual rate at which the electrode is fed into the slag pool with a required electrode feed rate which will give a shallow dish shaped molten metal pool at the top of the ingot and providing an error signal indicative of the difference between the two rates of feed; and e. means for varying the applied voltage in response to the error signal so that the electrode is consumed at a rate which maintains both the required feed rate and the predetermined separation between the lower end of the electrode and the top of the ingot.
 8. Electro-slag apparatus as claimed in claim 7, wherein the means for comparing the actual electrode feed rate with a required electrode feed rate and providing an error signal comprise a differential gear arrangement having two rotatable members driven at speeds corresponding to the actual feed rate and the required feed rate respectively and arranged to drive a third rotatable member at a speed corresponding to the difference between the two rates of feed.
 9. Electro-slag apparatus as claimed in claim 7, wherein the means for comparing the actual electrode feed rate with a required electrode feed rate and providing an error signal comprise two devices arranged to generate electrical signals indicative of the actual feed rate and the required feed rate respectively, and a further device arranged to compare the two electrical signals and produce a third electrical signal indicative of the difference between the two rates of feed.
 10. Electro-slag apparatus as claimed in claim 7, wherein the means for varying the relative movement between the electrode and the mold comprise means for providing a first signal indicative of the voltage between the lower end of the electrode and the upper end of the ingot, means for providing a second signal indicative of the current flowing between the electrode and the ingot, and means for combining the first and second signals and producing an output signal indicative of the electrical impedance between the lower end of the electrode and the top of the growing ingot, whIch output signal is utilized to vary the relative movement between the electrode and the mold. 